Conductive polymer composite

ABSTRACT

A conductive polymer composite is disclosed. The composite comprises a thermoplastic polymer; carbon nanotubes in an amount ranging from 2% to about 40% by weight, relative to the total weight of the conductive polymer composite; and a plurality of graphitic particles in an amount ranging from about 2% to about 50% by weight, relative to the total weight of the conductive polymer composite.

DETAILED DESCRIPTION

Field of the Disclosure

The present disclosure is directed to a conductive polymer composite.

Background

Additive manufacturing (also known as three dimensional printing) aspracticed in industry has been, to date, mostly concerned with printingstructural features. There is a need for materials and processes thatintegrate functional properties (such as electronic features) intoadditive manufacturing. Recently, conductive materials that arepotentially useful in additive manufacturing have been commercialized,but their conductivities are generally low, ranging from ˜10⁻³ S/cm toupwards of ˜2.0 S/cm. The mechanical properties of the commerciallyavailable materials, particularly the conductive materials such asAcrylonitrile butadiene styrene (ABS) or polylactic acid (PLA), aregenerally limited (e.g., are not flexible and/or are fairly brittle),which limits use as a conductive component.

There is great interest in the field of additive manufacturing todevelop improved materials that can be used to easily print completelyintegrated functional objects with limited post-assembly. This wouldallow completely new designs in the manufacturing and consumption ofeveryday objects, particularly when they can be enabled with conductivematerials. The capability of printing conductive components within anobject can provide the potential for embedded sensors and electronics.

Common techniques in additive manufacturing utilize the extrusion ofmolten polymer through a heated nozzle. This method is used in, forexample, fused deposition modeling (FDM), where a filament is fed into ahot zone for continuous extrusion. The molten polymer can be depositedlayer by layer onto a build plate in order to form 3D objects. There arevery few filament materials currently on the market which exhibitelectrical conductivity, and those which are available have relativelylow conductivities, which limits the range of potential applications.The materials are typically constructed such that one conductivematerial forms a percolating network through an insulating polymer base,such that electrons have a continuous pathway to flow. The formation ofthis conductive network is limited to the way the conductive particlesare arranged within the polymer base. Although these materials have beenextensively explored in both academia and industry, the focus istypically on minimizing the amount of conductive additive required toform a percolating network, where the conductivity is relatively low.One example of a paper directed to the study of electrical percolationis Yao Sun et al., Modeling of the Electrical Percolation of MixedCarbon Fillers in Polymer-Based Composites, Macromolecules 2009, 42,459-463, which describes the use of multi-walled carbon nanotubes andeither carbon black or graphite to lower percolation thresholds forpolymer composites. This paper does not describe techniques forincreasing conductivity substantially beyond the percolation threshold.Nor does it discuss the use of conductive polymers for additivemanufacturing.

Novel plastic composite materials that exhibit increased conductivitywould be a welcome step forward in the art, and could have significantimpacts in the field of additive manufacturing.

SUMMARY

An embodiment of the present disclosure is directed to a conductivepolymer composite. The composite comprises a thermoplastic polymer;carbon nanotubes in an amount ranging from 2% to about 40% by weight,relative to the total weight of the conductive polymer composite; and aplurality of graphitic particles in an amount ranging from about 2% toabout 50% by weight, relative to the total weight of the conductivepolymer composite.

Another embodiment of the present disclosure is directed to a method ofthree dimensional printing. The method comprises providing a compositeto a three-dimensional printer. The composite comprises a thermoplasticpolymer, carbon nanotubes in an amount ranging from 2% to about 20% byweight relative to the total weight of the conductive polymer composite,and a plurality of graphitic particles in an amount ranging from about2% to about 50% by weight relative to the total weight of the conductivepolymer composite. The composite is heated and the heated composite isextruded onto a build platform to form a three dimensional object.

Yet another embodiment is directed to a conductive polymer compositefilament. The filament comprises a thermoplastic polymer; carbonnanotubes in an amount ranging from 2% to about 40% by weight, relativeto the total weight of the conductive polymer composite; and a pluralityof graphitic particles in an amount ranging from about 2% to about 50%by weight, relative to the total weight of the conductive polymercomposite.

The compositions of the present application exhibit one or more of thefollowing advantages: improved conductivity of filaments for 3D printingapplications, such as fused deposition modeling (FDM); an unexpected,synergistic increase in electrical conductivity when secondary,graphitic fillers are added to multi-walled carbon nanotube/polymercomposites; or an improved method for increasing the electricalconductivity in polymer composites while retaining material propertiessuitable for additive manufacturing.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the present teachings, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentteachings and together with the description, serve to explain theprinciples of the present teachings.

FIG. 1 illustrates a three-dimensional printer employing a filament madewith the compositions of the present disclosure.

FIG. 2 shows a synergistic effect of multi-walled carbon nanotubes andgraphene in a thermoplastic polymer base, according to an example of thepresent disclosure.

FIG. 3 shows a synergistic effect of multi-walled carbon nanotubes andgraphite in a thermoplastic polymer base, according to an example of thepresent disclosure.

FIG. 4 shows conductivities of various example compositions, accordingto the present disclosure.

It should be noted that some details of the figure have been simplifiedand are drawn to facilitate understanding of the embodiments rather thanto maintain strict structural accuracy, detail, and scale.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the presentteachings, examples of which are illustrated in the accompanyingdrawings. In the drawings, like reference numerals have been usedthroughout to designate identical elements. In the followingdescription, reference is made to the accompanying drawings that form apart thereof, and in which is shown by way of illustration a specificexemplary embodiment in which the present teachings may be practiced.The following description is, therefore, merely exemplary.

An embodiment of the present disclosure is directed to a conductivepolymer composite. The composite comprises a thermoplastic polymer,carbon nanotubes and a plurality of graphitic particles. The carbonnanotubes are in an amount ranging from 2% to about 40% by weight,relative to the total weight of the conductive polymer composite. Theplurality of graphitic particles are in an amount ranging from about 2%to about 50% by weight, relative to the total weight of the conductivepolymer composite. The term “graphitic particles” is defined herein toinclude both graphene particles and graphite particles.

Any suitable thermoplastic polymer useful in three dimensional printingcan be employed in the composites of the present disclosure. Thecomposite can include a single polymer or mixtures of thermoplasticpolymers, including mixtures of any of the thermoplastic polymersdisclosed herein. In an embodiment, the thermoplastic polymer comprisesat least one repeating unit selected from the group consisting ofacrylate units, carboxylic acid ester units, amide units, lactic acidunits, benzimidazole units, carbonate ester units, ether units, sulfoneunits, arylketone units, arylether units, etherimide units, ethyleneunits, phenylene oxide units, propylene units, styrene units, vinylhalide units and carbamate units. In an embodiment, the thermoplasticpolymer is a copolymer, such as a block copolymer, of two or more of anyof the above listed repeating units. As an example, the thermoplasticpolymer can comprise at least one polymer selected from the groupconsisting of polyacrylates, polybenzimidazoles, polycarbonates,polyether sulfones, polyaryl ether ketones such as polyether etherketone, polyetherimide, polyethylenes such as polyethylene andpoly(ethylene-co-vinylacetate), polyphenylene oxides, polypropylenessuch as polypropylene and Poly(vinylidenefluoride-co-hexafluoropropylene), polystyrenes such as polystyrene,poly(styrene isoprene styrene), acrylonitrile butadiene styrene (ABS)and poly(Styrene Ethylene Butylene Styrene) (SEBS), polyesters such aspolyethylene terephthalate, polylactic acid (PLA) and polycaprolactone,polyurethanes, polyamides such as nylon, Poly(vinylidene fluoride)(PVDF) and polyvinyl chlorides. In an embodiment, the thermoplasticpolymer does not include Acrylonitrile butadiene styrene (ABS) or PLA.

In an embodiment, the thermoplastic polymer is a selected from the groupconsisting of polyacrylates and copolymer of acrylates, such as blockcopolymers of acrylates. The acrylate copolymers can comprise at leastone acrylate monomer and optionally one or more additional monomers suchas any of those monomers listed above for use in the thermoplasticpolymers. Such polymers can be formulated to have a desired degree offlexibility. In an embodiment, the polymer can be a polyester, such aspolycaprolactone.

The thermoplastic polymer can be included in the composite in anysuitable amount that will allow the composite to function in a threedimensional printing process. Examples of suitable amounts include arange of from about 10% to about 90% by weight, such as about 40% toabout 70%, or about 40% to about 60%, relative to the total weight ofthe conductive polymer composite.

The composite can include carbon nanotubes and graphitic particles inany suitable amount that will provide the desired conductivity. Exampleamounts of carbon nanotubes include a range of from 2% to about 40% byweight, such as about 5% to about 20% or about 5% to about 15%, relativeto the total weight of the conductive polymer composite. Larger amountsof carbon nanotubes may reduce processability of the composition by a 3Dprinter, depending, on among other things, the type of thermoplastic andthe printing process employed. Thus, in an embodiment, carbon nanotubeconcentrations of 20% by weight or less, such as 10% by weight or less,may be preferred. Example amounts of graphitic particles include a rangeof from about 10% to about 50% by weight, or about 15% to about 40% byweight, or about 20% to about 40% by weight, or about 25% to about 40%by weight, or about 30% to about 35% by weight, relative to the totalweight of the conductive polymer composite.

Any suitable carbon nanotubes can be employed. Examples of suitablecarbon nanotubes include single walled carbon nanotubes, multi-walledcarbon nanotubes and mixtures thereof. In an embodiment, the carbonnanotubes are multi-walled carbon nanotubes. Commercially availablesources of carbon nanotubes include, for example, carbon nanotubesavailable from CHEAPTUBES™ or NANOCYL™, such as Nanocyl 7000.

Any suitable graphitic particles can be employed in the composites ofthe present disclosure. The graphitic particles can be selected fromgraphene particles, graphite particles and mixtures of grapheneparticles and graphite particles.

The conductive polymer composites of the present disclosure can includeany other suitable optional ingredients in any desired amounts, such ascarrier liquids, plasticizers, dispersants and surfactants.Alternatively, ingredients not expressly recited in the presentdisclosure can be limited and/or excluded from the conductive polymercomposites disclosed herein. Thus, the amounts of the thermoplasticpolymer, carbon nanotubes and graphitic particles, with or without anyoptional ingredients as recited herein such as carrier liquids,plasticizers, dispersants and/or surfactants, can add up to 90% to 100%by weight of the total ingredients employed in the composites of thepresent disclosure, such as 95% to 100% by weight, or 98% to 100% byweight, or 99% to 100% by weight, or 100% by weight of the totalingredients.

The composite of the present disclosure can be in any suitable form. Inan embodiment, the composite is a conductive paste. The paste can be apaste at room temperature or a material that needs to be heated in orderto flow like a paste. In an embodiment, the paste comprises at least onecarrier liquid. In an embodiment, the carrier liquid may be a solventcapable of dissolving one or more of the paste ingredients. In anotherembodiment, the carrier liquid is not a solvent. Suitable carrierliquids for the paste include, for example, toluene, pyrrolidones (e.g.N-methylpyrrolidone, 1-cyclohexyl-2-pyrrolidone), N,N-dimethylformamide(DMF), N,N-dimethylacetamide dimethylsulfoxide andhexamethylphosphoramide. The carrier liquid can be included in the pastein any suitable amount, such as, for example, about 0.5% to about 60%weight percent based on the total weight of the wet composite paste.Optional additives that can be included in the paste are, for example,dispersants, surfactants, other solvents in addition to the carrierliquid and other conductive additives.

In an alternative embodiment, the composite can be in the form of a drycomposite having less than 5% liquid carrier, such as less than 3%, 2%or 1 liquid carrier by weight relative to the total weight of the drycomposite, such as no liquid carrier. The dry composite can be formedusing solvent, which is then removed by any suitable method, such as byheating, vacuum and/or other liquid removal techniques. Alternatively,the composite can be made without carrier liquid using neat processingtechniques.

The composite has a bulk conductivity greater than 1 S/cm, such asgreater than 3 S/cm, such as greater than 3.5 S/cm or greater than 4S/cm. Bulk conductivity is calculated using the formula,

σ=L/(R*A)  (1)

Where:

-   -   σ is bulk electrical conductivity;    -   L is length of the filament;    -   R is measured resistance of an extruded filament;    -   A is the cross-sectional area (πr²) of the filament, where r is        the radius of the filament.        The resistance, R, can be measured by forming an extruded        filament made from the composite. The tips of the filament are        painted with silver to provide good electrical connections with        the testing equipment (e.g., a digital multimeter), but would        not necessarily be painted if the filaments were to be used in        additive manufacturing. Resistance can then be measured across        the length of the filament. The dimensions of the filament and        the measured value for R can then be used to calculate bulk        conductivity (a) of the composite.

The composites of the present disclosure can be made by any suitablemethod. For example, the thermoplastic polymer can be combined with thecarbon nanotubes and graphitic particles using melt mixing techniques.Other suitable techniques for mixing such compositions are well known inthe art.

The present disclosure is also directed to a method of three dimensionalprinting. Any type of three dimensional printing can be employed, suchas filament printing (e.g., FDM) or paste extrusion. The method includesproviding any of the conductive polymer composites of the presentdisclosure to a three dimensional printer. The composite can be in anysuitable form useful in three dimensional printing, such as a filamentor paste. The conductive polymer can be heated to a molten statesuitable for extrusion. Then the heated conductive polymer is extrudedonto a substrate to form a three dimensional object.

The conductive polymer composites of the present disclosure can be usedin a FDM process by first forming the composite into a filament having adesired shape and dimensions (e.g., by extrusion or any other suitableprocess). The filament can have any suitable shape that will allow thefilament to be loaded into a 3 D FDM printer and printed. The filament,as initially supplied, can have a continuous length that is much longerthan its thickness, T, (shown in FIG. 1) such as a ratio of length tothickness that is greater than 100 to 1, such as greater than 500 to 1or 1000 to 1 or more, where T is the smallest thickness dimension of thefilament (e.g., the diameter if the filament has a circularcross-section). Any suitable thickness can be used, and may depend onthe 3D printer being used. As an example, thicknesses can range fromabout 0.1 mm to about 10 mm, such as about 0.5 mm to about 5 mm, orabout 1 mm to about 3 mm.

An example of a three dimensional printer 100 employing a filament ofthe present disclosure is shown in FIG. 1. The three dimensional printer100 includes a feeder mechanism 102 for supplying the filament 104 to aliquifier 106. The liquifier 106 melts the filament 104 and theresulting molten plastic is extruded through a nozzle 108 and depositedon a build platform 110. The feeder mechanism 102 can comprise rollersor any other suitable mechanism capable of supplying the filament 104from, for example, a spool of filament (not shown). The liquifier 106can employ any technique for heating the filament, such as heatingelements, lasers and so forth. The three dimensional printer 100 asshown in FIG. 1 is exemplary only and any type of three dimensionalprinter can be employed to deposit the filaments of the presentdisclosure.

EXAMPLES Example 1

Conductive polymer composites were prepared by melt mixing a polymerbase (Polycaprolactone) with 10% by weight of multi-walled carbonnanotubes (MWNT) and 30% by weight graphene on a Haake twin-screwextruder for 30 minutes at 30 rpm. The resulting material wascryogenically ground and the ground composite was extruded into afilament using a Melt Flow Indexer (MFI) and a modified die. Theconditions for extrusion on the MFI included a 1.8 mm orifice and 16.96kg weight (the weight on the MFI provides the force for extrusion) inorder to prepare the final filament. The final filament had a diameterof about 1.75 mm.

Example 2

A 10 cm section of the extruded filament of Example 1, with ends paintedin silver paint, were used to measure resistance in order to calculatebulk conductivity. Resistance measurements were completed using adigital multimeter. Bulk conductivity was calculated as about 3.9 S/cmusing formula 1 above.

Comparative Example A

A composite similar to that of Example 1 was made, but without graphene.

Comparative Example B

A composite similar to that of Example 1 was made, but without themulti-walled carbon nanotubes.

Example 3

A composite similar to that of Example 1 was made, but with 30% byweight graphite instead of the graphene.

Example 4

A composite similar to that of Example 1 was made, but with 10% byweight graphite.

Comparative Example C

A composite similar to that of Example 3 was made, but without MWNT andusing 20% by weight of graphite.

Example 5

A composite similar to that of Example 1 was made, but with 5% by weightMWNT and 30% by weight graphite in place of the graphene.

Example 6

A composite similar to that of Example 1 was made, but with 5% by weightMWNT and 30% by weight graphene. The graphene had a particle size ofabout 25 microns.

Example 7

A composite similar to that of Example 1 was made, but with 5% by weightMWNT and 30% by weight graphene. The graphene had a particle size ofabout 5 microns.

Bulk conductivity was measured for each of the examples and comparativeexamples above in a manner similar to that described for Example 2.Results for Examples 1 and 2 and Comparative Examples A and B are shownin FIG. 2. Results for Examples 3 and 4 and Comparative Examples A and Care shown in FIG. 3. Results for Examples 5 to 7 are shown in FIG. 4.

It is noted that the 10% by weight MWNT carbon nanotube concentrationsin Comparative Example A resulted in a conductivity of 0.51 S/cm, whichis well above the percolation threshold. Thus, the graphitic particleswere added to MWNT-polymer system that has already reached itspercolation threshold.

From the results for graphene in FIG. 2, it was evident that thecombination of MWNT and graphene had a synergistic effect when combinedwith one another in a plastic composite, since the combination had amuch higher conductivity than each of the components on their own. Theresults for graphite in FIG. 3 show a similar synergistic effect. With10% graphite/10% MWNT loading the conductivity was approximately doubledcompared with the MWNT alone. Increasing the loading to 30% graphite/10%MWNT the conductivity is 8 times higher than just the MWNT networkalone. Such a large increase in conductivity would not have beenexpected.

Comparing the results in FIG. 4 to those of FIGS. 2 and 3 appears toshow that compositions with about 5% by weight MWNT had significantlyincreased conductivity compared with similar compositions having 10% byweight MWNT. For instance, the compositions with about 5% by weight MWNTand 30% by weight graphene in FIG. 4 each had a conductivity of 4.7,whereas a similar composition with 10% by weight MWNT and 30% graphenehad a conductivity of 3.9, as shown in FIG. 2. The composition withabout 5% by weight MWNT and 30% by weight graphite of FIG. 4 had aconductivity of 6.2, whereas a similar composition with 10% by weightMWNT and 30% graphite had a conductivity of 4.2, as shown in FIG. 3.

It was not expected that a synergistic increase in conductivity wouldoccur for the Example compositions of FIGS. 2 to 4 for several reasons.The graphitic particles on their own have significantly lowerconductivity varying by one or two orders of magnitude. Also, it was notevident that the increase in graphitic particle loading would result insuch a drastic increase in conductivity at the carbon nanotubeconcentrations employed. It also was not expected that higherconductivities would be achieved for the compositions having lowerMWNTs, as reported in FIG. 4.

Thus, the data of FIGS. 2 to 4 demonstrate that an unexpected nonlinearincrease in conductivity was observed upon addition of a secondarygraphitic particle, which evidences a synergistic effect of thecombination carbon nanotubes and graphitic particles at relatively highloadings. This synergistic increase provides an additional advantage inthe case of additive manufacturing because increasing the loading of asingle particle would not be an effective method for increasing theconductivity. In the case of MWNT, for example, a maximum loading ofabout 20% by weight is reached where the composite it no longerprocessable for at least some additive manufacturing techniques orprocesses. At this loading the melt flow temperature exceeds thecapabilities of current technologies. In addition, the graphiticparticles exhibit conductivity orders of magnitude lower (10⁻²-10⁻³S/cm) compared to the conductivity of the MWNT. Only the combinationdemonstrates a nonlinear increase in electrical conductivity in thecomposite material, while retaining the processability desired foradditive manufacturing technologies.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all sub-ranges subsumedtherein.

While the present teachings have been illustrated with respect to one ormore implementations, alterations and/or modifications can be made tothe illustrated examples without departing from the spirit and scope ofthe appended claims. In addition, while a particular feature of thepresent teachings may have been disclosed with respect to only one ofseveral implementations, such feature may be combined with one or moreother features of the other implementations as may be desired andadvantageous for any given or particular function. Furthermore, to theextent that the terms “including,” “includes,” “having,” “has,” “with,”or variants thereof are used in either the detailed description and theclaims, such terms are intended to be inclusive in a manner similar tothe term “comprising.” Further, in the discussion and claims herein, theterm “about” indicates that the value listed may be somewhat altered, aslong as the alteration does not result in nonconformance of the processor structure to the illustrated embodiment. Finally, “exemplary”indicates the description is used as an example, rather than implyingthat it is an ideal.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompasses by the following claims.

What is claimed is:
 1. A conductive polymer composite, comprising: a thermoplastic polymer; carbon nanotubes in an amount ranging from 2% to about 40% by weight, relative to the total weight of the conductive polymer composite; and a plurality of graphitic particles in an amount ranging from about 2% to about 50% by weight, relative to the total weight of the conductive polymer composite.
 2. The composite of claim 1, wherein the thermoplastic polymer comprises at least one repeating unit selected from the group consisting of acrylate units, carboxylic acid ester units, amide units, lactic acid units, benzimidazole units, carbonate ester units, ether units, sulfone units, arylketone units, arylether units, arylalkyl units, etherimide units, ethylene units, phenylene oxide units, propylene units, styrene units, vinyl halide units and carbamate units.
 3. The composite of claim 2, wherein the thermoplastic polymer is a copolymer of two or more of the repeating units.
 4. The composite of claim 3, wherein the copolymer comprises one or more acrylate units.
 5. The composite of claim 1, wherein the thermoplastic polymer comprises at least one polymer selected from the group consisting of polyacrylates, polybenzimidazoles, polycarbonates, polyether sulfones, polyaryl ether ketones, polyethylenes, polyphenylene oxides, polypropylenes, polystyrenes, polyesters, polyurethanes, polyamides, Poly(vinylidene fluoride) (PVDF) and polyvinyl chlorides, polyether ether ketone, poly(ethylene-co-vinylacetate), polyetherimide, polypropylene, Poly(vinylidene fluoride-co-hexafluoropropylene), poly(styrene isoprene styrene), acrylonitrile butadiene styrene (ABS), poly(Styrene Ethylene Butylene Styrene) (SEBS), polyethylene terephthalate, polylactic acid (PLA), polycaprolactone and nylon.
 6. The composite of claim 1, wherein the thermoplastic polymer is in an amount ranging from about 10% to about 90% by weight, relative to the total weight of the conductive polymer composite.
 7. The composite of claim 1, wherein the carbon nanotubes are multi-walled carbon nanotubes.
 8. The composite of claim 1, wherein the carbon nanotubes are in an amount ranging from about 5% to about 20% by weight, relative to the total weight of the conductive polymer composite.
 9. The composite of claim 1, wherein the graphitic particles comprise at least one material selected from the group consisting of graphene and graphite.
 10. The composite of claim 1, wherein the graphitic particles are in an amount ranging from about 10% to about 50% by weight, relative to the total weight of the conductive polymer composite.
 11. The composite of claim 1, wherein the graphitic particles are graphene particles.
 12. The composite of claim 1, wherein the graphitic particles are graphite particles.
 13. The composite of claim 1, wherein the composite has a bulk conductivity greater than 1 S/cm, where the conductivity is calculated using the formula σ=L/(R*A), based on the measured resistance (R) of an extruded filament made from the composite and having silver painted tips, the filament having a length (L) of 10 cm and a diameter of 1.75 mm.
 14. A method of three dimensional printing, the method comprising: providing a composite to a three-dimensional printer, the composite comprising a thermoplastic polymer, carbon nanotubes in an amount ranging from 2% to about 20% by weight relative to the total weight of the conductive polymer composite, and a plurality of graphitic particles in an amount ranging from about 2% to about 50% by weight relative to the total weight of the conductive polymer composite; heating the composite; and extruding the heated composite onto a build platform to form a three dimensional object.
 15. The method of claim 14, wherein the heated composite is in the form of a filament.
 16. The method of claim 14, wherein the graphitic particles comprise at least one material selected from the group consisting of graphene and graphite.
 17. A conductive polymer composite filament, comprising: a thermoplastic polymer; carbon nanotubes in an amount ranging from 2% to about 40% by weight, relative to the total weight of the conductive polymer composite; and a plurality of graphitic particles in an amount ranging from about 2% to about 50% by weight, relative to the total weight of the conductive polymer composite.
 18. The conductive polymer composite filament of claim 17, wherein the thermoplastic polymer comprises at least one repeating unit selected from the group consisting of acrylate units, carboxylic acid ester units, amide units, lactic acid units, benzimidazole units, carbonate ester units, ether units, sulfone units, arylketone units, arylether units, arylalkyl units, etherimide units, ethylene units, phenylene oxide units, propylene units, styrene units, vinyl halide units and carbamate units.
 19. The conductive polymer composite filament of claim 17, wherein the thermoplastic polymer comprises at least one polymer selected from the group consisting of polyacrylates, polybenzimidazoles, polycarbonates, polyether sulfones, polyaryl ether ketones, polyethylenes, polyphenylene oxides, polypropylenes, polystyrenes, polyesters, polyurethanes, polyamides, Poly(vinylidene fluoride) (PVDF) and polyvinyl chlorides, polyether ether ketone, poly(ethylene-co-vinylacetate), polyetherimide, polypropylene, Poly(vinylidene fluoride-co-hexafluoropropylene), poly(styrene isoprene styrene), acrylonitrile butadiene styrene (ABS), poly(Styrene Ethylene Butylene Styrene) (SEBS), polyethylene terephthalate, polylactic acid (PLA), polycaprolactone and nylon.
 20. The conductive polymer composite filament of claim 17, wherein the graphitic particles comprise at least one material selected from the group consisting of graphene and graphite. 